AC vs DC Digital Drives

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AC vs DC Digital Drives

Should I specify an AC drive or a DC drive in my application?

In short, consider the total cost and performance benefits of each. AC is perceived as cheaper because AC motors are cheaper than DC motors. The total installed cost and performance may be quite different:

CONSIDER	AC	DC
System Purchase Price	More attractive in some power ranges.	Best at high power or very low power.
Drive Purchase Price and Maintenance	Usually more expensive using the latest technology.	Uses well proven technology that is cost effective.
Motor Purchase Price and Maintenance	Less expensive and more reliable.	More costly due to Commutator and brush technology. Accessory faces readily available.
Installation Cost	Several subtle issues may drive cost up.	Usually less expensive and straightforward.
Overhauling Load (Braking)	RED ALERT	Easily accommodated
Efficiency	Two power conversion stages.	One stage conversion
Power Factor	Constant and better.	OK at high speed
Harmonics	6, 12, 18, 24 pulse converters available at a cost.	6 or 12 pulse converter
Performance	Servo like	Lower bandwidth

System Purchase Price

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It is usually the case that the combined cost of a DC drive and motor is lower for very small HP applications; AC wins from 5HP to about 100HP, then DC wins again as power increases. If you already have a DC motor, then DC is probably going to cost less.

Drive Purchase and Maintenance Costs

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The �brains� of all drives, AC and DC drives, cost about the same amount and use similar processors. The power and firing electronics dictate the total drive cost.

DC drives use SCRs to handle the switching power. AC drives use IGBTs as switching elements. Fewer IGBTs are required in a power bridge, but they are much more costly than SCRs, especially at higher power levels.

AC drives also include an AC to DC conversion section. This uses diodes and bus capacitors not necessary in a DC drive. These capacitors are expensive and become a maintenance issue over time.

Motor Purchase and Maintenance Costs

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AC motors are usually less expensive than DC motors because they are simpler. AC motors don�t have brushes like DC motors. Brush maintenance cost is usually sited in any cost comparison. The only good thing about DC brush maintenance is that it can be scheduled and usually doesn�t result in unexpected down time.

Installation Cost

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For multiple drives, common bus AC drives can save money on system installation since only one mains connection is required vs. individual DC drive mains connections. AC motors require only three power conductors; DC motors require four. However, most AC drive suppliers recommend special shielded power cables to minimize electrical interference from the AC drive. A motor power lead filter that may be required if the cables to AC motor are over 100 ft. long. Also, "special designed EMI filters may be required to be installed on the input side of the AC drive when it is installed in close proximity to EMI sensitive instrumentation devices. Add up all the extras when comparing installation costs.

Braking

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Does the application require the drive to provide braking torque? If so, how much and how often? Even a little braking torque can be an issue if it is continuous. It adds up.

DC drives can easily pump braking energy back to the AC mains. In fact the cost for full regeneration is so reasonable that Avtron offers regenerative braking bridges for free up to about 300HP. Above that non-regenerative DC bridges are available and cost less than their AC drive counterparts. Alternatively, dynamic braking can be provided by adding an external DB contactor and DB resistors. The advantage goes to DC.

AC drives are more complex. Braking energy goes back to the DC link (bus) inside the drive. This makes the DC voltage rise. If it rises too much, the drive trips off line. There must be a way to absorb this energy before that occurs. This can be by DB resistor and DB chopper circuit or by more complex means described next.

AC drives may be stand alone or common bus. Most are stand alone drives that are powered from the AC mains. (Each drive is powered separately.) This type of drive usually relies on DB resistors to absorb braking energy. Care must be exercised to specify the maximum braking and duty cycle to avoid overheating the resistor or the DB chopper in the drive.

Common bus AC drives are available, and can use braking energy more efficiently. They are powered by a common DC bus with one AC power supply. Several common bus AC drives are tied to the same DC bus and can dissipate braking power among them. This works so long as the combined sum of all the drives is always drawing energy from the AC mains. Emergency stop may require DB resistors. A more complex and expensive power supply can be purchased that regenerates back to the AC mains line. Alternatively, a regenerative DC drive or for even more money an inverter can be used to regenerate to the line for improved harmonics.

In short, AC drive applications that require braking cost more than DC applications, and require more complex components that fail more often.

Efficiency

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DC drives are more energy-efficient. AC drives have two stages of power conversion (AC to DC then DC to AC). DC drives have one stage (AC to DC). Each stage has energy losses, in the form of heat generated in the drive. More heat is generated during switching (switching losses). The higher the switching frequency, the more the losses. If braking is required, the regeneration available in most DC drives will increase efficiency further beyond the DB resistors used on most AC drives With their additional stages and higher switching frequencies, AC drives generate more heat than DC, and are less efficient.

Power Factor

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AC drives have a better power factor in most applications. DC drives approach the same power factor as AC drives only if you operate the DC drive at or near maximum speed.

Harmonics

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AC and DC drives often produce similar harmonic problems on the AC mains. In either case harmonics can be reduced by going to a 12 pulse (six phase) rectification scheme. This requires a phase-shifting transformer. The cost added for a 12 pulse AC drive is less than it is for the 12 pulse DC drive. AC drives can also be purchased in 18 or 24 pulse configurations. An active front end AC drive which utilizes an inverter for the DC bus supply, though costly, has the lowest harmonic content. (What is an active front end, do most drives have them, and does that contradict the opening statement?)

Performance

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Higher switching frequency in the AC drive generally results in higher transient response capability than possible in a DC drive. AC motors permit to higher speeds. The AC motor may also be lower inertia. If the application requires �servo-like� performance or operation at high speed then AC is usually a better choice.

AC and DC can both be operated without an encoder. Performance suffers in either case. Low and zero speed performance is most severely affected.